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  <div class="section" id="module-weakref">
<span id="weakref-weak-references"></span><h1>8.8. <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal"><span class="pre">weakref</span></code></a> — Weak references<a class="headerlink" href="#module-weakref" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/weakref.py">Lib/weakref.py</a></p>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal"><span class="pre">weakref</span></code></a> module allows the Python programmer to create <em class="dfn">weak
references</em> to objects.</p>
<p>In the following, the term <em class="dfn">referent</em> means the object which is referred to
by a weak reference.</p>
<p>A weak reference to an object is not enough to keep the object alive: when the
only remaining references to a referent are weak references,
<a class="reference internal" href="../glossary.html#term-garbage-collection"><span class="xref std std-term">garbage collection</span></a> is free to destroy the referent and reuse its memory
for something else.  However, until the object is actually destroyed the weak
reference may return the object even if there are no strong references to it.</p>
<p>A primary use for weak references is to implement caches or
mappings holding large objects, where it’s desired that a large object not be
kept alive solely because it appears in a cache or mapping.</p>
<p>For example, if you have a number of large binary image objects, you may wish to
associate a name with each.  If you used a Python dictionary to map names to
images, or images to names, the image objects would remain alive just because
they appeared as values or keys in the dictionaries.  The
<a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakKeyDictionary</span></code></a> and <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakValueDictionary</span></code></a> classes supplied by
the <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal"><span class="pre">weakref</span></code></a> module are an alternative, using weak references to construct
mappings that don’t keep objects alive solely because they appear in the mapping
objects.  If, for example, an image object is a value in a
<a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakValueDictionary</span></code></a>, then when the last remaining references to that
image object are the weak references held by weak mappings, garbage collection
can reclaim the object, and its corresponding entries in weak mappings are
simply deleted.</p>
<p><a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakKeyDictionary</span></code></a> and <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakValueDictionary</span></code></a> use weak references
in their implementation, setting up callback functions on the weak references
that notify the weak dictionaries when a key or value has been reclaimed by
garbage collection.  <a class="reference internal" href="#weakref.WeakSet" title="weakref.WeakSet"><code class="xref py py-class docutils literal"><span class="pre">WeakSet</span></code></a> implements the <a class="reference internal" href="stdtypes.html#set" title="set"><code class="xref py py-class docutils literal"><span class="pre">set</span></code></a> interface,
but keeps weak references to its elements, just like a
<a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakKeyDictionary</span></code></a> does.</p>
<p><a class="reference internal" href="#weakref.finalize" title="weakref.finalize"><code class="xref py py-class docutils literal"><span class="pre">finalize</span></code></a> provides a straight forward way to register a
cleanup function to be called when an object is garbage collected.
This is simpler to use than setting up a callback function on a raw
weak reference, since the module automatically ensures that the finalizer
remains alive until the object is collected.</p>
<p>Most programs should find that using one of these weak container types
or <a class="reference internal" href="#weakref.finalize" title="weakref.finalize"><code class="xref py py-class docutils literal"><span class="pre">finalize</span></code></a> is all they need – it’s not usually necessary to
create your own weak references directly.  The low-level machinery is
exposed by the <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal"><span class="pre">weakref</span></code></a> module for the benefit of advanced uses.</p>
<p>Not all objects can be weakly referenced; those objects which can include class
instances, functions written in Python (but not in C), instance methods, sets,
frozensets, some <a class="reference internal" href="../glossary.html#term-file-object"><span class="xref std std-term">file objects</span></a>, <a class="reference internal" href="../glossary.html#term-generator"><span class="xref std std-term">generator</span></a>s, type
objects, sockets, arrays, deques, regular expression pattern objects, and code
objects.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.2: </span>Added support for thread.lock, threading.Lock, and code objects.</p>
</div>
<p>Several built-in types such as <a class="reference internal" href="stdtypes.html#list" title="list"><code class="xref py py-class docutils literal"><span class="pre">list</span></code></a> and <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal"><span class="pre">dict</span></code></a> do not directly
support weak references but can add support through subclassing:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Dict</span><span class="p">(</span><span class="nb">dict</span><span class="p">):</span>
    <span class="k">pass</span>

<span class="n">obj</span> <span class="o">=</span> <span class="n">Dict</span><span class="p">(</span><span class="n">red</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">green</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">blue</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span>   <span class="c1"># this object is weak referenceable</span>
</pre></div>
</div>
<p>Other built-in types such as <a class="reference internal" href="stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal"><span class="pre">tuple</span></code></a> and <a class="reference internal" href="functions.html#int" title="int"><code class="xref py py-class docutils literal"><span class="pre">int</span></code></a> do not support weak
references even when subclassed (This is an implementation detail and may be
different across various Python implementations.).</p>
<p>Extension types can easily be made to support weak references; see
<a class="reference internal" href="../extending/newtypes.html#weakref-support"><span class="std std-ref">Weak Reference Support</span></a>.</p>
<dl class="class">
<dt id="weakref.ref">
<em class="property">class </em><code class="descclassname">weakref.</code><code class="descname">ref</code><span class="sig-paren">(</span><em>object</em><span class="optional">[</span>, <em>callback</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.ref" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a weak reference to <em>object</em>.  The original object can be retrieved by
calling the reference object if the referent is still alive; if the referent is
no longer alive, calling the reference object will cause <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal"><span class="pre">None</span></code></a> to be
returned.  If <em>callback</em> is provided and not <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal"><span class="pre">None</span></code></a>, and the returned
weakref object is still alive, the callback will be called when the object is
about to be finalized; the weak reference object will be passed as the only
parameter to the callback; the referent will no longer be available.</p>
<p>It is allowable for many weak references to be constructed for the same object.
Callbacks registered for each weak reference will be called from the most
recently registered callback to the oldest registered callback.</p>
<p>Exceptions raised by the callback will be noted on the standard error output,
but cannot be propagated; they are handled in exactly the same way as exceptions
raised from an object’s <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal"><span class="pre">__del__()</span></code></a> method.</p>
<p>Weak references are <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a> if the <em>object</em> is hashable.  They will
maintain their hash value even after the <em>object</em> was deleted.  If
<a class="reference internal" href="functions.html#hash" title="hash"><code class="xref py py-func docutils literal"><span class="pre">hash()</span></code></a> is called the first time only after the <em>object</em> was deleted,
the call will raise <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal"><span class="pre">TypeError</span></code></a>.</p>
<p>Weak references support tests for equality, but not ordering.  If the referents
are still alive, two references have the same equality relationship as their
referents (regardless of the <em>callback</em>).  If either referent has been deleted,
the references are equal only if the reference objects are the same object.</p>
<p>This is a subclassable type rather than a factory function.</p>
<dl class="attribute">
<dt id="weakref.ref.__callback__">
<code class="descname">__callback__</code><a class="headerlink" href="#weakref.ref.__callback__" title="Permalink to this definition">¶</a></dt>
<dd><p>This read-only attribute returns the callback currently associated to the
weakref.  If there is no callback or if the referent of the weakref is
no longer alive then this attribute will have value <code class="docutils literal"><span class="pre">None</span></code>.</p>
</dd></dl>

<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.4: </span>Added the <a class="reference internal" href="#weakref.ref.__callback__" title="weakref.ref.__callback__"><code class="xref py py-attr docutils literal"><span class="pre">__callback__</span></code></a> attribute.</p>
</div>
</dd></dl>

<dl class="function">
<dt id="weakref.proxy">
<code class="descclassname">weakref.</code><code class="descname">proxy</code><span class="sig-paren">(</span><em>object</em><span class="optional">[</span>, <em>callback</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.proxy" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a proxy to <em>object</em> which uses a weak reference.  This supports use of
the proxy in most contexts instead of requiring the explicit dereferencing used
with weak reference objects.  The returned object will have a type of either
<code class="docutils literal"><span class="pre">ProxyType</span></code> or <code class="docutils literal"><span class="pre">CallableProxyType</span></code>, depending on whether <em>object</em> is
callable.  Proxy objects are not <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a> regardless of the referent; this
avoids a number of problems related to their fundamentally mutable nature, and
prevent their use as dictionary keys.  <em>callback</em> is the same as the parameter
of the same name to the <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-func docutils literal"><span class="pre">ref()</span></code></a> function.</p>
</dd></dl>

<dl class="function">
<dt id="weakref.getweakrefcount">
<code class="descclassname">weakref.</code><code class="descname">getweakrefcount</code><span class="sig-paren">(</span><em>object</em><span class="sig-paren">)</span><a class="headerlink" href="#weakref.getweakrefcount" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the number of weak references and proxies which refer to <em>object</em>.</p>
</dd></dl>

<dl class="function">
<dt id="weakref.getweakrefs">
<code class="descclassname">weakref.</code><code class="descname">getweakrefs</code><span class="sig-paren">(</span><em>object</em><span class="sig-paren">)</span><a class="headerlink" href="#weakref.getweakrefs" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a list of all weak reference and proxy objects which refer to <em>object</em>.</p>
</dd></dl>

<dl class="class">
<dt id="weakref.WeakKeyDictionary">
<em class="property">class </em><code class="descclassname">weakref.</code><code class="descname">WeakKeyDictionary</code><span class="sig-paren">(</span><span class="optional">[</span><em>dict</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakKeyDictionary" title="Permalink to this definition">¶</a></dt>
<dd><p>Mapping class that references keys weakly.  Entries in the dictionary will be
discarded when there is no longer a strong reference to the key.  This can be
used to associate additional data with an object owned by other parts of an
application without adding attributes to those objects.  This can be especially
useful with objects that override attribute accesses.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Caution: Because a <a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakKeyDictionary</span></code></a> is built on top of a Python
dictionary, it must not change size when iterating over it.  This can be
difficult to ensure for a <a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakKeyDictionary</span></code></a> because actions
performed by the program during iteration may cause items in the
dictionary to vanish “by magic” (as a side effect of garbage collection).</p>
</div>
</dd></dl>

<p><a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakKeyDictionary</span></code></a> objects have an additional method that
exposes the internal references directly.  The references are not guaranteed to
be “live” at the time they are used, so the result of calling the references
needs to be checked before being used.  This can be used to avoid creating
references that will cause the garbage collector to keep the keys around longer
than needed.</p>
<dl class="method">
<dt id="weakref.WeakKeyDictionary.keyrefs">
<code class="descclassname">WeakKeyDictionary.</code><code class="descname">keyrefs</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakKeyDictionary.keyrefs" title="Permalink to this definition">¶</a></dt>
<dd><p>Return an iterable of the weak references to the keys.</p>
</dd></dl>

<dl class="class">
<dt id="weakref.WeakValueDictionary">
<em class="property">class </em><code class="descclassname">weakref.</code><code class="descname">WeakValueDictionary</code><span class="sig-paren">(</span><span class="optional">[</span><em>dict</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakValueDictionary" title="Permalink to this definition">¶</a></dt>
<dd><p>Mapping class that references values weakly.  Entries in the dictionary will be
discarded when no strong reference to the value exists any more.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Caution:  Because a <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakValueDictionary</span></code></a> is built on top of a Python
dictionary, it must not change size when iterating over it.  This can be
difficult to ensure for a <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakValueDictionary</span></code></a> because actions performed
by the program during iteration may cause items in the dictionary to vanish “by
magic” (as a side effect of garbage collection).</p>
</div>
</dd></dl>

<p><a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakValueDictionary</span></code></a> objects have an additional method that has the
same issues as the <code class="xref py py-meth docutils literal"><span class="pre">keyrefs()</span></code> method of <a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakKeyDictionary</span></code></a>
objects.</p>
<dl class="method">
<dt id="weakref.WeakValueDictionary.valuerefs">
<code class="descclassname">WeakValueDictionary.</code><code class="descname">valuerefs</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakValueDictionary.valuerefs" title="Permalink to this definition">¶</a></dt>
<dd><p>Return an iterable of the weak references to the values.</p>
</dd></dl>

<dl class="class">
<dt id="weakref.WeakSet">
<em class="property">class </em><code class="descclassname">weakref.</code><code class="descname">WeakSet</code><span class="sig-paren">(</span><span class="optional">[</span><em>elements</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakSet" title="Permalink to this definition">¶</a></dt>
<dd><p>Set class that keeps weak references to its elements.  An element will be
discarded when no strong reference to it exists any more.</p>
</dd></dl>

<dl class="class">
<dt id="weakref.WeakMethod">
<em class="property">class </em><code class="descclassname">weakref.</code><code class="descname">WeakMethod</code><span class="sig-paren">(</span><em>method</em><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakMethod" title="Permalink to this definition">¶</a></dt>
<dd><p>A custom <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-class docutils literal"><span class="pre">ref</span></code></a> subclass which simulates a weak reference to a bound
method (i.e., a method defined on a class and looked up on an instance).
Since a bound method is ephemeral, a standard weak reference cannot keep
hold of it.  <a class="reference internal" href="#weakref.WeakMethod" title="weakref.WeakMethod"><code class="xref py py-class docutils literal"><span class="pre">WeakMethod</span></code></a> has special code to recreate the bound
method until either the object or the original function dies:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="gp">... </span>    <span class="k">def</span> <span class="nf">method</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span>        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;method called!&quot;</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">r</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">ref</span><span class="p">(</span><span class="n">c</span><span class="o">.</span><span class="n">method</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">r</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">r</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">WeakMethod</span><span class="p">(</span><span class="n">c</span><span class="o">.</span><span class="n">method</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">r</span><span class="p">()</span>
<span class="go">&lt;bound method C.method of &lt;__main__.C object at 0x7fc859830220&gt;&gt;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">r</span><span class="p">()()</span>
<span class="go">method called!</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">c</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">gc</span><span class="o">.</span><span class="n">collect</span><span class="p">()</span>
<span class="go">0</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">r</span><span class="p">()</span>
<span class="go">&gt;&gt;&gt;</span>
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.4.</span></p>
</div>
</dd></dl>

<dl class="class">
<dt id="weakref.finalize">
<em class="property">class </em><code class="descclassname">weakref.</code><code class="descname">finalize</code><span class="sig-paren">(</span><em>obj</em>, <em>func</em>, <em>*args</em>, <em>**kwargs</em><span class="sig-paren">)</span><a class="headerlink" href="#weakref.finalize" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a callable finalizer object which will be called when <em>obj</em>
is garbage collected. Unlike an ordinary weak reference, a finalizer
will always survive until the reference object is collected, greatly
simplifying lifecycle management.</p>
<p>A finalizer is considered <em>alive</em> until it is called (either explicitly
or at garbage collection), and after that it is <em>dead</em>.  Calling a live
finalizer returns the result of evaluating <code class="docutils literal"><span class="pre">func(*arg,</span> <span class="pre">**kwargs)</span></code>,
whereas calling a dead finalizer returns <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal"><span class="pre">None</span></code></a>.</p>
<p>Exceptions raised by finalizer callbacks during garbage collection
will be shown on the standard error output, but cannot be
propagated.  They are handled in the same way as exceptions raised
from an object’s <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal"><span class="pre">__del__()</span></code></a> method or a weak reference’s
callback.</p>
<p>When the program exits, each remaining live finalizer is called
unless its <a class="reference internal" href="atexit.html#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-attr docutils literal"><span class="pre">atexit</span></code></a> attribute has been set to false.  They
are called in reverse order of creation.</p>
<p>A finalizer will never invoke its callback during the later part of
the <a class="reference internal" href="../glossary.html#term-interpreter-shutdown"><span class="xref std std-term">interpreter shutdown</span></a> when module globals are liable to have
been replaced by <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal"><span class="pre">None</span></code></a>.</p>
<dl class="method">
<dt id="weakref.finalize.__call__">
<code class="descname">__call__</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.finalize.__call__" title="Permalink to this definition">¶</a></dt>
<dd><p>If <em>self</em> is alive then mark it as dead and return the result of
calling <code class="docutils literal"><span class="pre">func(*args,</span> <span class="pre">**kwargs)</span></code>.  If <em>self</em> is dead then return
<a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal"><span class="pre">None</span></code></a>.</p>
</dd></dl>

<dl class="method">
<dt id="weakref.finalize.detach">
<code class="descname">detach</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.finalize.detach" title="Permalink to this definition">¶</a></dt>
<dd><p>If <em>self</em> is alive then mark it as dead and return the tuple
<code class="docutils literal"><span class="pre">(obj,</span> <span class="pre">func,</span> <span class="pre">args,</span> <span class="pre">kwargs)</span></code>.  If <em>self</em> is dead then return
<a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal"><span class="pre">None</span></code></a>.</p>
</dd></dl>

<dl class="method">
<dt id="weakref.finalize.peek">
<code class="descname">peek</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.finalize.peek" title="Permalink to this definition">¶</a></dt>
<dd><p>If <em>self</em> is alive then return the tuple <code class="docutils literal"><span class="pre">(obj,</span> <span class="pre">func,</span> <span class="pre">args,</span>
<span class="pre">kwargs)</span></code>.  If <em>self</em> is dead then return <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal"><span class="pre">None</span></code></a>.</p>
</dd></dl>

<dl class="attribute">
<dt id="weakref.finalize.alive">
<code class="descname">alive</code><a class="headerlink" href="#weakref.finalize.alive" title="Permalink to this definition">¶</a></dt>
<dd><p>Property which is true if the finalizer is alive, false otherwise.</p>
</dd></dl>

<dl class="attribute">
<dt id="weakref.finalize.atexit">
<code class="descname">atexit</code><a class="headerlink" href="#weakref.finalize.atexit" title="Permalink to this definition">¶</a></dt>
<dd><p>A writable boolean property which by default is true.  When the
program exits, it calls all remaining live finalizers for which
<a class="reference internal" href="#weakref.finalize.atexit" title="weakref.finalize.atexit"><code class="xref py py-attr docutils literal"><span class="pre">atexit</span></code></a> is true.  They are called in reverse order of
creation.</p>
</dd></dl>

<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">It is important to ensure that <em>func</em>, <em>args</em> and <em>kwargs</em> do
not own any references to <em>obj</em>, either directly or indirectly,
since otherwise <em>obj</em> will never be garbage collected.  In
particular, <em>func</em> should not be a bound method of <em>obj</em>.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.4.</span></p>
</div>
</dd></dl>

<dl class="data">
<dt id="weakref.ReferenceType">
<code class="descclassname">weakref.</code><code class="descname">ReferenceType</code><a class="headerlink" href="#weakref.ReferenceType" title="Permalink to this definition">¶</a></dt>
<dd><p>The type object for weak references objects.</p>
</dd></dl>

<dl class="data">
<dt id="weakref.ProxyType">
<code class="descclassname">weakref.</code><code class="descname">ProxyType</code><a class="headerlink" href="#weakref.ProxyType" title="Permalink to this definition">¶</a></dt>
<dd><p>The type object for proxies of objects which are not callable.</p>
</dd></dl>

<dl class="data">
<dt id="weakref.CallableProxyType">
<code class="descclassname">weakref.</code><code class="descname">CallableProxyType</code><a class="headerlink" href="#weakref.CallableProxyType" title="Permalink to this definition">¶</a></dt>
<dd><p>The type object for proxies of callable objects.</p>
</dd></dl>

<dl class="data">
<dt id="weakref.ProxyTypes">
<code class="descclassname">weakref.</code><code class="descname">ProxyTypes</code><a class="headerlink" href="#weakref.ProxyTypes" title="Permalink to this definition">¶</a></dt>
<dd><p>Sequence containing all the type objects for proxies.  This can make it simpler
to test if an object is a proxy without being dependent on naming both proxy
types.</p>
</dd></dl>

<dl class="exception">
<dt id="weakref.ReferenceError">
<em class="property">exception </em><code class="descclassname">weakref.</code><code class="descname">ReferenceError</code><a class="headerlink" href="#weakref.ReferenceError" title="Permalink to this definition">¶</a></dt>
<dd><p>Exception raised when a proxy object is used but the underlying object has been
collected.  This is the same as the standard <a class="reference internal" href="exceptions.html#ReferenceError" title="ReferenceError"><code class="xref py py-exc docutils literal"><span class="pre">ReferenceError</span></code></a> exception.</p>
</dd></dl>

<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0205"><strong>PEP 205</strong></a> - Weak References</dt>
<dd>The proposal and rationale for this feature, including links to earlier
implementations and information about similar features in other languages.</dd>
</dl>
</div>
<div class="section" id="weak-reference-objects">
<span id="weakref-objects"></span><h2>8.8.1. Weak Reference Objects<a class="headerlink" href="#weak-reference-objects" title="Permalink to this headline">¶</a></h2>
<p>Weak reference objects have no methods and no attributes besides
<a class="reference internal" href="#weakref.ref.__callback__" title="weakref.ref.__callback__"><code class="xref py py-attr docutils literal"><span class="pre">ref.__callback__</span></code></a>. A weak reference object allows the referent to be
obtained, if it still exists, by calling it:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">weakref</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">Object</span><span class="p">:</span>
<span class="gp">... </span>    <span class="k">pass</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">o</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">r</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">ref</span><span class="p">(</span><span class="n">o</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">o2</span> <span class="o">=</span> <span class="n">r</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">o</span> <span class="ow">is</span> <span class="n">o2</span>
<span class="go">True</span>
</pre></div>
</div>
<p>If the referent no longer exists, calling the reference object returns
<a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal"><span class="pre">None</span></code></a>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">o</span><span class="p">,</span> <span class="n">o2</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">r</span><span class="p">())</span>
<span class="go">None</span>
</pre></div>
</div>
<p>Testing that a weak reference object is still live should be done using the
expression <code class="docutils literal"><span class="pre">ref()</span> <span class="pre">is</span> <span class="pre">not</span> <span class="pre">None</span></code>.  Normally, application code that needs to use
a reference object should follow this pattern:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="c1"># r is a weak reference object</span>
<span class="n">o</span> <span class="o">=</span> <span class="n">r</span><span class="p">()</span>
<span class="k">if</span> <span class="n">o</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
    <span class="c1"># referent has been garbage collected</span>
    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Object has been deallocated; can&#39;t frobnicate.&quot;</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Object is still live!&quot;</span><span class="p">)</span>
    <span class="n">o</span><span class="o">.</span><span class="n">do_something_useful</span><span class="p">()</span>
</pre></div>
</div>
<p>Using a separate test for “liveness” creates race conditions in threaded
applications; another thread can cause a weak reference to become invalidated
before the weak reference is called; the idiom shown above is safe in threaded
applications as well as single-threaded applications.</p>
<p>Specialized versions of <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-class docutils literal"><span class="pre">ref</span></code></a> objects can be created through subclassing.
This is used in the implementation of the <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal"><span class="pre">WeakValueDictionary</span></code></a> to reduce
the memory overhead for each entry in the mapping.  This may be most useful to
associate additional information with a reference, but could also be used to
insert additional processing on calls to retrieve the referent.</p>
<p>This example shows how a subclass of <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-class docutils literal"><span class="pre">ref</span></code></a> can be used to store
additional information about an object and affect the value that’s returned when
the referent is accessed:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">weakref</span>

<span class="k">class</span> <span class="nc">ExtendedRef</span><span class="p">(</span><span class="n">weakref</span><span class="o">.</span><span class="n">ref</span><span class="p">):</span>
    <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">ob</span><span class="p">,</span> <span class="n">callback</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">annotations</span><span class="p">):</span>
        <span class="nb">super</span><span class="p">(</span><span class="n">ExtendedRef</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="n">ob</span><span class="p">,</span> <span class="n">callback</span><span class="p">)</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">__counter</span> <span class="o">=</span> <span class="mi">0</span>
        <span class="k">for</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">annotations</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
            <span class="nb">setattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="sd">&quot;&quot;&quot;Return a pair containing the referent and the number of</span>
<span class="sd">        times the reference has been called.</span>
<span class="sd">        &quot;&quot;&quot;</span>
        <span class="n">ob</span> <span class="o">=</span> <span class="nb">super</span><span class="p">(</span><span class="n">ExtendedRef</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="fm">__call__</span><span class="p">()</span>
        <span class="k">if</span> <span class="n">ob</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">__counter</span> <span class="o">+=</span> <span class="mi">1</span>
            <span class="n">ob</span> <span class="o">=</span> <span class="p">(</span><span class="n">ob</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">__counter</span><span class="p">)</span>
        <span class="k">return</span> <span class="n">ob</span>
</pre></div>
</div>
</div>
<div class="section" id="example">
<span id="weakref-example"></span><h2>8.8.2. Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2>
<p>This simple example shows how an application can use object IDs to retrieve
objects that it has seen before.  The IDs of the objects can then be used in
other data structures without forcing the objects to remain alive, but the
objects can still be retrieved by ID if they do.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">weakref</span>

<span class="n">_id2obj_dict</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">WeakValueDictionary</span><span class="p">()</span>

<span class="k">def</span> <span class="nf">remember</span><span class="p">(</span><span class="n">obj</span><span class="p">):</span>
    <span class="n">oid</span> <span class="o">=</span> <span class="nb">id</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span>
    <span class="n">_id2obj_dict</span><span class="p">[</span><span class="n">oid</span><span class="p">]</span> <span class="o">=</span> <span class="n">obj</span>
    <span class="k">return</span> <span class="n">oid</span>

<span class="k">def</span> <span class="nf">id2obj</span><span class="p">(</span><span class="n">oid</span><span class="p">):</span>
    <span class="k">return</span> <span class="n">_id2obj_dict</span><span class="p">[</span><span class="n">oid</span><span class="p">]</span>
</pre></div>
</div>
</div>
<div class="section" id="finalizer-objects">
<span id="finalize-examples"></span><h2>8.8.3. Finalizer Objects<a class="headerlink" href="#finalizer-objects" title="Permalink to this headline">¶</a></h2>
<p>The main benefit of using <a class="reference internal" href="#weakref.finalize" title="weakref.finalize"><code class="xref py py-class docutils literal"><span class="pre">finalize</span></code></a> is that it makes it simple
to register a callback without needing to preserve the returned finalizer
object.  For instance</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">weakref</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">Object</span><span class="p">:</span>
<span class="gp">... </span>    <span class="k">pass</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">kenny</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="n">kenny</span><span class="p">,</span> <span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;You killed Kenny!&quot;</span><span class="p">)</span>  
<span class="go">&lt;finalize object at ...; for &#39;Object&#39; at ...&gt;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">kenny</span>
<span class="go">You killed Kenny!</span>
</pre></div>
</div>
<p>The finalizer can be called directly as well.  However the finalizer
will invoke the callback at most once.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">def</span> <span class="nf">callback</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span><span class="p">):</span>
<span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;CALLBACK&quot;</span><span class="p">)</span>
<span class="gp">... </span>    <span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span> <span class="o">+</span> <span class="n">z</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">obj</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">f</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">callback</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="n">z</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="n">f</span><span class="o">.</span><span class="n">alive</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="n">f</span><span class="p">()</span> <span class="o">==</span> <span class="mi">6</span>
<span class="go">CALLBACK</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="ow">not</span> <span class="n">f</span><span class="o">.</span><span class="n">alive</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">f</span><span class="p">()</span>                     <span class="c1"># callback not called because finalizer dead</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">obj</span>                 <span class="c1"># callback not called because finalizer dead</span>
</pre></div>
</div>
<p>You can unregister a finalizer using its <a class="reference internal" href="#weakref.finalize.detach" title="weakref.finalize.detach"><code class="xref py py-meth docutils literal"><span class="pre">detach()</span></code></a>
method.  This kills the finalizer and returns the arguments passed to
the constructor when it was created.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">obj</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">f</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">callback</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="n">z</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">f</span><span class="o">.</span><span class="n">detach</span><span class="p">()</span>                                           
<span class="go">(&lt;__main__.Object object ...&gt;, &lt;function callback ...&gt;, (1, 2), {&#39;z&#39;: 3})</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">newobj</span><span class="p">,</span> <span class="n">func</span><span class="p">,</span> <span class="n">args</span><span class="p">,</span> <span class="n">kwargs</span> <span class="o">=</span> <span class="n">_</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="ow">not</span> <span class="n">f</span><span class="o">.</span><span class="n">alive</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="n">newobj</span> <span class="ow">is</span> <span class="n">obj</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="n">func</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span> <span class="o">==</span> <span class="mi">6</span>
<span class="go">CALLBACK</span>
</pre></div>
</div>
<p>Unless you set the <a class="reference internal" href="#weakref.finalize.atexit" title="weakref.finalize.atexit"><code class="xref py py-attr docutils literal"><span class="pre">atexit</span></code></a> attribute to
<a class="reference internal" href="constants.html#False" title="False"><code class="xref py py-const docutils literal"><span class="pre">False</span></code></a>, a finalizer will be called when the program exits if it
is still alive.  For instance</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">obj</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;obj dead or exiting&quot;</span><span class="p">)</span>  
<span class="go">&lt;finalize object at ...; for &#39;Object&#39; at ...&gt;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">exit</span><span class="p">()</span>                                               
<span class="go">obj dead or exiting</span>
</pre></div>
</div>
</div>
<div class="section" id="comparing-finalizers-with-del-methods">
<h2>8.8.4. Comparing finalizers with <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal"><span class="pre">__del__()</span></code></a> methods<a class="headerlink" href="#comparing-finalizers-with-del-methods" title="Permalink to this headline">¶</a></h2>
<p>Suppose we want to create a class whose instances represent temporary
directories.  The directories should be deleted with their contents
when the first of the following events occurs:</p>
<ul class="simple">
<li>the object is garbage collected,</li>
<li>the object’s <code class="xref py py-meth docutils literal"><span class="pre">remove()</span></code> method is called, or</li>
<li>the program exits.</li>
</ul>
<p>We might try to implement the class using a <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal"><span class="pre">__del__()</span></code></a> method as
follows:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">TempDir</span><span class="p">:</span>
    <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">tempfile</span><span class="o">.</span><span class="n">mkdtemp</span><span class="p">()</span>

    <span class="k">def</span> <span class="nf">remove</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
            <span class="n">shutil</span><span class="o">.</span><span class="n">rmtree</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="kc">None</span>

    <span class="nd">@property</span>
    <span class="k">def</span> <span class="nf">removed</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="ow">is</span> <span class="kc">None</span>

    <span class="k">def</span> <span class="nf">__del__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">remove</span><span class="p">()</span>
</pre></div>
</div>
<p>Starting with Python 3.4, <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal"><span class="pre">__del__()</span></code></a> methods no longer prevent
reference cycles from being garbage collected, and module globals are
no longer forced to <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal"><span class="pre">None</span></code></a> during <a class="reference internal" href="../glossary.html#term-interpreter-shutdown"><span class="xref std std-term">interpreter shutdown</span></a>.
So this code should work without any issues on CPython.</p>
<p>However, handling of <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal"><span class="pre">__del__()</span></code></a> methods is notoriously implementation
specific, since it depends on internal details of the interpreter’s garbage
collector implementation.</p>
<p>A more robust alternative can be to define a finalizer which only references
the specific functions and objects that it needs, rather than having access
to the full state of the object:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">TempDir</span><span class="p">:</span>
    <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">tempfile</span><span class="o">.</span><span class="n">mkdtemp</span><span class="p">()</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">_finalizer</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">shutil</span><span class="o">.</span><span class="n">rmtree</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">remove</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">_finalizer</span><span class="p">()</span>

    <span class="nd">@property</span>
    <span class="k">def</span> <span class="nf">removed</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">_finalizer</span><span class="o">.</span><span class="n">alive</span>
</pre></div>
</div>
<p>Defined like this, our finalizer only receives a reference to the details
it needs to clean up the directory appropriately. If the object never gets
garbage collected the finalizer will still be called at exit.</p>
<p>The other advantage of weakref based finalizers is that they can be used to
register finalizers for classes where the definition is controlled by a
third party, such as running code when a module is unloaded:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">weakref</span><span class="o">,</span> <span class="nn">sys</span>
<span class="k">def</span> <span class="nf">unloading_module</span><span class="p">():</span>
    <span class="c1"># implicit reference to the module globals from the function body</span>
<span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">modules</span><span class="p">[</span><span class="vm">__name__</span><span class="p">],</span> <span class="n">unloading_module</span><span class="p">)</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">If you create a finalizer object in a daemonic thread just as the program
exits then there is the possibility that the finalizer
does not get called at exit.  However, in a daemonic thread
<a class="reference internal" href="atexit.html#atexit.register" title="atexit.register"><code class="xref py py-func docutils literal"><span class="pre">atexit.register()</span></code></a>, <code class="docutils literal"><span class="pre">try:</span> <span class="pre">...</span> <span class="pre">finally:</span> <span class="pre">...</span></code> and <code class="docutils literal"><span class="pre">with:</span> <span class="pre">...</span></code>
do not guarantee that cleanup occurs either.</p>
</div>
</div>
</div>


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  <ul>
<li><a class="reference internal" href="#">8.8. <code class="docutils literal"><span class="pre">weakref</span></code> — Weak references</a><ul>
<li><a class="reference internal" href="#weak-reference-objects">8.8.1. Weak Reference Objects</a></li>
<li><a class="reference internal" href="#example">8.8.2. Example</a></li>
<li><a class="reference internal" href="#finalizer-objects">8.8.3. Finalizer Objects</a></li>
<li><a class="reference internal" href="#comparing-finalizers-with-del-methods">8.8.4. Comparing finalizers with <code class="docutils literal"><span class="pre">__del__()</span></code> methods</a></li>
</ul>
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